The invention relates generally to an apparatus for immobilization of the spine, and more particularly, to transpedicular screws.
Various methods of spinal immobilization have been known and used during this century in the treatment of spinal instability and displacement. The preferred treatment for spinal stabilization is immobilization of the joint by surgical fusion, or arthrodesis. This method has been known since its development in 1911 by Hibbs and Albee. However, in many cases, and in particular, in cases involving fusion across the lumbosacral articulation and when there are many levels involved, pseudoarthrosis is a problem. It was discovered that immediate immobilization was necessary in order to allow a bony union to form. Early in the century, post operative external immobilization such as the use of splints and casts was the favored methods of treatment, however, as surgical techniques have become more sophisticated, various methods of internal and external fixation have been developed.
Internal fixation refers to methods of stabilization which are wholly internal to the patient and include commonly known devices such as bone plates and pins. External fixation in contrast involves at least some portion of the stabilization device which is external to the patient""s body. Internal fixation is now the favored method of immobilization since the patient is allowed greater freedom with the elimination of the external portion of the device. Moreover, the possibility of infections, such as pin tract infection, is reduced.
Some of the indications treated by internal fixation of the spine include vertebral displacement and management such as kyphosis, spondylolishtesis and rotation; segmental instability, such as disc degeneration and fracture caused by disease and trauma and congenital defects; and tumor diseases.
A common problem with spinal fixation is the question of how to secure the fixation device to the spine without damaging the spinal cord. The pedicles are a favored area of attachment since they offer an area that is strong enough to hold the fixation device even when the patient suffers from osteoporosis. Since the middle 1950""s, methods of fixation have utilized the pedicles. In early methods, screws were extended through the facets into the pedicles. Subsequently, posterior methods of fixation have been developed which utilize wires that extend through the spinal canal and hold a rod against the lamina (such as the Luque system) or that use pedicular screws that extend into the pedicle and secure a plate extending across several vertebral segments (such as the Steffe plate).
There are problems of fixation unique to this area of the spine such as the fact that the lumbar spine is normally lordotic and this lordosis must be preserved. In addition, indicated spinal decompression often requires a destabilization of the spine posteriorly. This may result in instability unless fusion is used, and fusion will often fail to become solid unless effective internal fixation is used. Finally, the points of sacral fixation are the weakest point of fixation.
Prior art devices for spinal fixation are discussed above as including the Steffe plate and the Luque System. A complete discussion of various internal fixation devices are included in L. Wiltse, xe2x80x9cInternal Fixation of the Lumbar Spinexe2x80x9d, Clinical Orthopaedics and Related Research, February. 1986, No. 203, p.p. 2-219. Known implant configurations include facet screws, double distraction systems, compression distraction systems, springs, spinous process plates, wired implants and transpedicular screw and plate systems.
Common distraction and compression systems utilize a threaded rod and hooks which engage selected transverse processes of the vertebrae. Examples of such systems include the Harrington distraction system sold by Zimmer USA, Inc., the Keene system shown in U.S. Pat. No. 4,269,178 and the Lewis-Greenlaw System illustrated in U.S. Pat. No. 4,085,744. U.S. Pat. No. 3,648,691 to Lumb et al. shows the use of spinous process plates.
Wired implants are favored by some orthopedic surgeons because of the flexibility of the system. Dr. Eduardo Luque has developed a wired implant system where two L-shaped rods are secured along their long sides to the vertebral laminae by means of wires which pass through the vertebral foramina. The short legs of the rods extend across the vertebrae between the spinous process. A similar wired implant is shown in U.S. Pat. No. 4,604,995 to Stephens et al.
Transpedicular screw and plate systems rely on a screw threaded into a reamed canal or hole generally positioned perpendicular to the longitudinal axis of the spine and horizontal or parallel to the anterior/posterior plane of the vertebral body. Methyl methacrylate is sometimes used to secure the screw in the canal, particularly if osteoporosis is a problem. The screws engage a plate which has been bent to conform to the normal curvature of the spine or to the points of desired reduction. One such screw and plate system which has been used with significant success is the Steffee system. In this system, the screws are inserted first, the spine plates are then inserted over the pedicle screws and then posterior tapered nuts are screwed on. The screws are tightened bilaterally until the plate is locked between two nuts.
While the wired implants have the advantages of facilitating vertebral alignment, thus permitting the device to allow for variations in individual spines, and decreasing rigidity, this method of fixation includes the increased risk of damage to the neural structures. This risk can be countered by the use of transpedicular screws and plates.
The pedicle presents an area for fixation of sufficient size and depth, that under careful conditions, the risk of damage to the nerve chord is reduced. On the other hand, the use of plates with the screws is more rigid than the wired implants and the tension and compression of the plate on the screw can cause dislocation or even shearing of the screw. In addition, the current plate designs are bulky and leave little surface for bone grafting and they cannot be contoured to any lateral curvature of the spine.
Puno et al, in U.S. Pat. No. 4,805,602, presented a new system sharing advantages of both the wired implants and the plate. Specifically, they taught a screw and rod system that provides a rigidity which is intermediate the wired implant and the plate systems. While the screw and rod system theretofore retained the stability provided by the plate and screw system, the system of Puno et al, could be contoured to any plane.
Puno, et al, in U.S. Pat. No. 5,360,431, disclosed improvements in their device that reduced the time required to perform the spinal operation as compared to the prior invention, from hours to around an hour. Such a time saving represents a significant reduction in the risk associated with a surgical procedure. Further, their new design was believed to be easier to use because the chances of cross-threading the nut unto the anchor are reduced and the nut is more accessible for tightening. This is of particular significance in the bloody environment which obscures the spinal surgeon""s access to the fixation device. The improved Puno et al device included a thin, chamfered nut to reduce bulk and yet includes a thread design to achieve sufficient compression on the rod. The anchor system presents a flush upper surface and each anchor seat is secured by a cancellous screw which cooperates through a sloped bore in the anchor seat so as to provide a limited ball and socket motion. The design of this system incorporates a method of therapy for treating a spinal indication utilizing this internal fixator.
For hollow screws that augment their holding power with an injectable cementitious fluid, in situ-setting calcium phosphate (Ca-P) cement and polymethyl methacrylate (PMMA) are used. Their ultimate pull-out strength of Ca-P cement- and PMMA-augmented bones is approximately the same.
Anterior cervical plate-screw fixation techniques were developed beginning in the 1960""s for a more direct fixation, in particular, to overcome progressive posterior protruding deformity, instability, and graft dislodgement in the treatment of variable conditions of the cervical spine. Hollow titanium screws were introduced in 1986 for solving fixation problems in vertebral bodies which consisted of mainly cancellous bone with very thin cortex. These vertebra have only weak holding power and, consequently, the screws may loosen over time resulting in hardware failure. Failure rates as high as 35% have been reported.
For stronger fixation, pedical screws have been used through posterior approach. This procedure is technically challenging but promising. In a cadaveric study using two different screw insertion techniquesxe2x80x94a xe2x80x9cwindowxe2x80x9d and a xe2x80x9cblindxe2x80x9d techniquexe2x80x94both techniques exhibited a high percentage of screws that violated the pedicle. In the xe2x80x9cwindowxe2x80x9d technique, a laminotomy xe2x80x9cwindowxe2x80x9d is created to determine in advance the superior, medial, and interior borders of the pedicle. A xe2x80x9cblindxe2x80x9d technique uses only the body topographical landmarks and predetermined 30xc2x0 medial and 20xc2x0 superior trajectory was associated with a violation rate of 47%. Although the xe2x80x9cwindowxe2x80x9d technique had the violation rate to 25%, neither technique was successful an acceptable number of times. Others have reported violation rates of 65.5%, 39.6% and 10.6% for methods using surface landmarks, laminoforminotomy and computer assisted guide systems.
Thus there remains a need for a better technique or system for fixation using transpedicular screws than that taught by the prior art and, in particular, one that does not carry with it an unacceptably high cortical violation rate.
To overcome the high cortical violation rate of exiting pedical screws, a self-guided transpedicular (or anterior pedical) screw has been developed. It has been noted that the cortex is harder than the interior portion of the pedical. The present invention takes advantage of this circumstance in its design. This screw comprises four parts: a head, a neck, a body and a tail. The head, or leading part, is dull and rounded so that it does not easily penetrate through the harder material such as the cortex of the pedicle but does penetrate softer material. The neck connected to the head is thinner than the head, sufficiently thin so that it is flexible. If the head encounters resistence from an angle, the neck will bend, thus allowing the head to change direction in response to the resistance. The body is hollow and treaded as with conventional transpedicular screws and the tail is formed to receive a tool for rotation of the screw about its long axis. The surface of the head is polished to a gloss so that it slides along a surface more easily when it encounters a hard surface at an angle.
A feature of the present invention is the rounded, dull head. This feature is designed to prevent cortical violation and facilitate sliding when it encounters greater resistence. Because the cortex of the pedicle is harder than its interior, the head will xe2x80x9cpreferxe2x80x9d to move in the direction of less resistence. Thus, it will remain in the softer interior of the pedicle.
Another feature of the present invention is the thinner neck of the transpedicular screw. This feature works in combination with the rounded head to allow the screw to bend relatively easily when it meets resistence rather than try to bore its way through the resistence or remain straight notwithstanding the change in direction.
Other features and their advantages will be apparent to those skilled in the art of orthopedic surgery and transpedicular screws from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings.